Method of and device for detecting physical quantities



1950 J. c. FROMMER 2,517,554

ms'mon OF AND DEVICE FOR DETECTING PHYSICAL QUANTITIES Filed June 21, 1949 7 Sheets-Sheet l JOSEPH C. FROM/15R IN VEN TOR.

Aug. 8, 1950 J. c. FROMMER 7 2,517,554

METHOD OF AND DEVICE FOR DETECTING PHYSICAL QUANTITIES Filed June 21. 1949 7 Sheets-Sheet 2 JOJEPH C. FROM/15R INVENTOR.

Aug. 8, 1950 J. c. FROMMER 2, METHOD OF AND DEVICE FOR DETECTING PHYSICAL QUANTITIES Filed June 21, 1949 '7 Sheets-Sheet 3 V V V V V V '0 JOSEPHC/WONNFR 5 INVENTOR.

A 8, 1950 J. c. FRQMMER 2,517,554

METHOD OF AND DEVICE FOR DETECTING PHYSICAL QUANTITIES Filed June 21, 1949 7 Sheets-Sheet 4 JOJEPH c. FR OMMER INVENTOR.

fl/for) Aug. 8, 1950 .1. c. FROMMER 2 ,55

METHOD OF AND DEVICE FOR DETECTING PHYSICAL QUANTITIES Filed June 21, 1949 7 Sheets-Sheet 5 J05PH C. FROM/YER INVENTOR.

hey

Aug. 8, 1950 J. c. FROMMER 2,517,554 um'ruon OF AND DEVICE FOR DETECTING v PHYSICAL QUANTITIES Filed June 21, 1949 7 Sheets-Sheet 6 JOSL-W/ c. FROM/15R IN V EN TOR.

Aug. 8, 1950 J c FQMMER 2 ,55

METHOD OF DEVICE FOR DETECTING PHYSICAL QUANTITIES Filed June 21, 1949 -7 Sheets-Sheet '7 v JOSPH a FROM/75R INVENTOR.

Patented Aug. 8, 1950 METHOD OF AND DEVICE FOR DETECTING PHYSICAL QUANTITIES oseph C. Frommer, Cincinnati, Ohio Application. June 2-1, 1949., Serial No. 100,496

. 17 Claims. 1

This invention relates to a method of detecting physical quantities, i. e. the quantitative value. of I physical. phenomena, which are capable of generating or influencing electric currents. Theinvention. also relates to apparatus for carrying out such methods.

The novel method is. basically one of comparison of two or ore. physical quantities which comparison, involves causing, a plurality of electric currents, including at least one current whose magnitude is a function of the physical quantity to be detected, to, flow simultaneously between the. electrodes ofa circuit element under such conditions that the passage of current through said circuit element will cause a voltage between said electrodes which is a logarithmic function oi the current so passing. At least one of said electric currentsv sent, simultaneously through the circuit element mentioned has to be inconstant in such a manner as to vary its instantaneous: value by an amplitude depending on the amplitude of the co-ordinated physical quantity. The voltage variations occurring between the electrodes of the. circuit element in consequence of, the variations of the instantaneous value of the mentioned inconstant current are utilized for an, indication of the physical quantity to be detected and it is to be noted that a. single such voltage variation may be sufilcient for causing the desired indication.

The invention takes advantage of the fact that the difference of the logarithms of two quantities is, the logarithm of their proportion, and that circuit elements are available, the voltage across which is a logarithm function of such current as passes between the electrodes of the circuit element within a certain current range. If the value of the current flowing across said circuit element changes, then the voltage variation is proportional to the logarithm of the proportion between the current before and the current after the change. If the proportion between the current before the change and the current after the changeis indicative of the relationship "between the two physical quantities to be compared, then the voltage variation occurring in connection with and at the rate of such current variations will equally be indicative of this relationship. These voltage variations I measure conveniently after suitable amplification, and use them for an indication of the relationship between the physical" quantities to be compared.

The general principle just referred; tois of very wide applicability as long as the physical quantig ties to be compared either directly or through some physical characteristics of them can influence electric currents. The physical quantity sought to be determined may be detected by comparison with another physical quantity of the same kind. For instance the light intensity of a. lamp in any direction may be compared. with the light intensity of a standardv lamp in. a. standard direction. Or, the physical quantity sought may be detected with reference. to a physical quantity of another kind. For instance, the. in:- tensity of a certain light flux may be determined with the aid of a capacitor by measuring the relative modulation obtained by superimposing the direct current flowing across a certain phototube under the action of said light flux. upon the alterna-ting current flowing in said capacitor. if the latter connected to a source of alternating voltage of a given frequency and given voltage.

Comparison between two physical quantities. of the-same kin-d can be expressed by pure numbers. The: comparisonbetween physical quantities: of different kinds will involvereference to some physical constants or to; physical properties of some pieces at equipment.

The above. mentioned logarithmic relationship requiredto exist between the voltage across the electrodes, of the circuit element and. the current flowing, through that element may be expressed as:

where I is the current flowing through the circuit element, V is the voltage across the circuit element, Ii1 is a constant (dimension currentl and V0 and U are other constants (dimension voltage), all depending on the nature of the circuit element used. The above expression can. be written:

VVo=U 10g(I/Io)-=U log I--U log" It Thus if the current in two diiferent instances assumesthe values Irand I2, the voltage will assume the values:

V1=Vo+U 10S Ii-U 10g 10;

and;

Vz=V'o-U log I2U1'0g I0 and the difference between the voltages in the two. instances will be V'rV2=U 10g I1U 10g I2=U 10g (II/I3) Thisxcondition is illustratedin. Fig. 15 of the drawings.

In this figure the curve S represents the logarithmic characteristics between current I and voltage V by its rectangular coordinates. It is seen that if the current varies from In to I21 by the amount Im, the voltage across the electrodes varies from V11 to V21 by Vm. And if the current varies from I12 to I22 by the same amount 1122:1111, then the voltage varies from V12 to V22 by V02, which is substantially smaller than V121. This is because even though ID2=ID1, the proportion between I132 and I12 is smallerthan the proportion between 1m and In.

The difference Im or Inz may symbolize one single step of current increase or current decrease, or the amplitude of modulation of a periodically varying current.

One of the currents flowing in the circuit element may be In and another I1 1. Bysuddenly interrupting Im (or by suddenly turning it on) We obtain a voltage step V131 which is significant of the ratio 1112121. The ratio of the two currents to be compared can be calculated by putting:

In other instances, one of the currents flowing in the circuit element may be a direct current 'of the magnitude (I11+I21)/2, and the other an alternating current of the peak to peak amplitude Im. Then the proportion between Ini and (I11+I 21) /2 can be calculated from I11:I 1 by put lting: v

In still other instances, one of the currents ma be a direct current of the magnitude In x and the other a current varying periodically between the values Il2-I21 and I22I21. Then, if we know the relative modulation of the second current, similar algebraic operations will yield the proportion between I21 and (Izz Izi) from the proportion 112:122 which can be determined, "according to the invention, fromVm. 4 j.

' The curve S of the drawing ha'sbeen obtained b parallel shifting the curve S in direction of the V axis It will be seen (andcan be proved mathematically) that the voltage variations V131 and V132 obtained with these shifted characteristics are equal to V121 and V132 obtained with the original characteristics for the same current variations.

The voltage resulting from the simultaneous application of the several currents upon one and the same circuit element having logarithmic characteristics may be used to actuate a signalling or controlling device. In some cases, however, it

may be desirable that'the comparison. yield numerical data which are identical to, proportional to, or known functions of the proportions between two physical quantities, or the magnitude of one of two physical quantities, while the other physg- If one of the physical quantitiesis ofdncon- 4 stant character, such as, e, g. the light emitted by a lamp fed by pulsating current, then the required variations of current are present without further steps. If, however, all the physical quantities to be compared are of constant character, then I cause at least one of them to vary its magnitude, periodically interrupt the action of one of them on the circuit element sensitive to it, or periodically interrupt the current of this circuitfelement For example, the photoelectric current created by a phototube under the influence of light emitted from a lamp fed by direct current (or by alternating current of such frequency that the fluctuations in light intensity are of no consequence) can be made inconstant in one of many diiferent ways which will be obvious to those skilled in the art, such as by interposing a light chopper between the lamp and the phototube, or by periodicall interrupting said photo-electric current, etc. It is also possible to cause just oneyariation of the physical quantity used as a reference, such as by turning onor off some physical action and detecting, the variation in voltage occurring, on the logarithmic. circuit element due to this variation. A readily accessible circuit element across which the voltage is a logarithmic function of the current in it is the path of an amplifier tube. between the thermionic cathode thereof andthe first of the other electrodes which surround the cathode. Ordinarily the first other electrode is the control grid of the tube. In usual vacuum tubes the current between the cathode and said first grid is temperaturelimited. if this first grid is left to assume a voltage somewhere between minus 2 and 0 volts with respect to the cathode. The voltagethen is very exactly a linear function of the logarithm of the, current between the two electrodes mentioned. If theinsulation is kept high, and the voltages on all other electrodes are kept low, say below 20 volts in ordinary radio tubes, or below 5 volts in special tubes built for operation at so low voltages and if other measures to keep grid leakage low, known to those skilled in theart, are followed, then the linear relationship between grid voltage and the logarithm of the cathodegrid current will be maintained even fforextremely low values of said current between cathode and grid.- This invention then also provides a novel method to measure extremely low direct currents by comparing them with alternating currents of known magnitude as will be shown in detail later. I a

The grid to cathode path of an amplifier tube is not the only circuit element across whichthe voltage is a logarithmicfunction of the current across it. Forv example, a diode operating in the temperature limited region, i e. at a voltage range of .say minus 2 to 0 volts between anode and cathode may serve the same purpose. Or certain portions of the current/voltage characteristics-of nonlinear resistors, as e, g. thyrite may also be used for the purpose of the invention.-, I It is well known that whereas thetemperature controlled current'adheres ver rigorously tothe exponential law, the contact potential between the surface of the grid and the rest of the circuit may shift these characteristics byseveral tenths of -arvolt. This shiftasks for recalibration inusual circ uit s,but does not aifect exact functioning ofthe circuits to be described, as is seen by comparison of voltage variations obtained along curve S with those obtained along curve Sin F 69 A. C a e in temperature. of the. cathode will change the exponent, in the logarithmic. function. The temperature. of the cathode, can be kept. onstant by voltage regulating transformers current regulating tubes. or other means. But-II have, found that the. changes. occurring. due to. normal variations in line voltag are of; little significance. and may be compensated in they am..- pli-fier by means that influence, the indication of thesystem in; opposite direction.

It is. also known, that, circuit. elements that in.- dicate; various; physical. quantities by proportional currents. e... g.. phototubes. may change. their. sensitivity with time. variations oi temperature, etc, It the physical quantities to. be compared are. of. the: same. kind, I preferably cause. them to act. on the same. such, circuit element, and, then such. variation. of sensitivity will not affect, the proportion. of currents caused by each of them and will in no way impair exact indication of their proportion by the circuits, according to the invention. y

The. invention will now be. explained with ref.- erence. to the accompanying drawings, in which;

Fig. 1 represents the circuit diagram. of a sim ple: but sensitive photoelectric relay circuit;-

Fig. 2 shows; a circuit suitable for, measuring extremely small currents. by a balance-method;

his a vector diagram; I

Fig. a shows a modified circuit for. measuring extremely small. currents Fig; 5. represents diagrammatically a. device. for measuring the. spectral composition of a source. of. light;

Fig; 6 shows the device of Fi 5 inv an end view; together with the diagram of acircuit tobe used with this device Figs. 7: ancl8 representtime curvesv of voltages obtained in the. device of: Fi s. 5and 6;

Fig; 9; illustrates diagrammatically a device for determinin quantitatively the. color of. a.refiect ing material;

Fig. 1.0 shows a. device, together with its circuit diagram, suitable to actuate a signal if th81t182115- parency of a, medium transgresses certain limits;

Fig; ll represents the plan section and circuit diagram of a device to measure transparency oil a medium;

Fig. 12 is a section along the. line t2:--l2 of Fig. I1;

Fig- 13 illustrates schematically a device for measuring ionization;

Fig. 14 shows in a side section a device to measure the absorption of gamma radiation, together with the diagram of a simple circuit for Fig. 15 is a plan view of the device shown in Fig.14; and l Fig. 16 represents logarithmic characteristics between current I and voltage V.

The device diagrammatically shownin Fig. I may serve the purpose of actuating a "relay 58, if the illumination of a phototube l ll surpasses a certain limit. Inthis figure; l0 denotes a photo.- tube surrounded by an electrostatic shield I and having a cathode I l and ananode i4. 20 denotes anamplifier tube having a cathode 2 t, a grid 22, a. screen grid 23-, and an anode 24. The cathode 2f of tube 20 is heated by a. filament which, as conventional in the illustrationoi indirectly heated cathodes, is not shown, as are not shown the otherfilaments in the drawing. The tube! 20 may also conveniently have athird or suppressor grid between the screen grid 23 and plate '24, this suppressor grid being connected to the cathode 6 2L internally or externally ct the tube. Suchoptional. suppressor grid is. not shown in the drawing neither for tube 20 nor for any other ampliner tube tobe mentioned hereafter. 40. denotes a. rectifier tube having v a cathode. 4| and a plate 44. 58 is an amplifier tube having a cathode. 5|, a grid 52, a screen grid 53. and a, plate 54. l,,, 2

and 3 are terminals of a supply of direct voltage,

I being the common. negative terminal, 2v a. terminal. supplying a voltage. of, say, volts, C. stabilized by a voltage. regulating tube, not shown, and 3 a terminal supplying say 300 volts D. C. which need not be. stabilized. 62 denotes a, secondary winding. of a transformer 60. only partly shown- The. winding 62 may be the filament winding of the power supply transformer of the amplifier or one. half of it. It is. connected between line l and one. terminal of. a variablecapacitor [5. The other terminal of the capacitor ii. is connected to the. grid 2-2 of tube. 20'. which is also connected to the cathode H of thephototube H1. The anode [4 of the phototube. I0 is connected to line 2. The cathode. 2 I of tube, 20 is connected to line I, itsscreen grid 23 is. connected across a capacitor 25 to line I and across a resistor 26 to line 2. Its anode 24 is connected across a resistor 21 to line 2 and across a capacitor. 2.8 to the cathode 4| of the rectifier 40. The rectifier cathode 4| is connected across a resistor 45.to the midpoint: of a. voltage divider 46,, 41, connected between the lines 3 and l. The plate 4.4 of. the rectifier 40, is connected to the; grid 52 of vacuum. tube 58,, and across a capacitor 48. which is in. parallel with. a resistor 56, tov line I. The

cathode 51 of the amplifier tube. 50 is; connected to line I, its screen grid 53' to line 2 and its plate 54 across. the. coil of arelay 58 to line 3. The only connection to cathode, ll of the phototube Ill being; the. capacitor 15, which can carry no direct current and the grid 22. of tube 20, all photoelectric current flowing from cathode H to anode 14 of the. phototube IO must. flow also from oath ode 211 to grid 22 of tube 20.. Due to the, logarithmic relationship between grid current and grid; voltage the conductance between grid and cathode is proportional to the grid, current. Now rid 22 lies. on the midpoint of a voltage. divider formed by the capacitor l5 and the. grid to cathode path, of, tube 20. This voltage divider is fed by the transformer winding 62 of the transformer 60. The A. C. voltage appearing between grid 22 and cathode 2| will be the greater the lower the grid to cathode. admittance. But the, grid. to cathode admittance is the greater the greater the. photoelectric current is. and therefore; the A. C voltage appearing between grid 22 and cathode 21 is indicative: of the photoelectric. current. The alternating voltage, appearing across the load re.- sistor' 2'1: depends on this A. C. voltage across 22', 2|- It: might also depend on the average (or, D. C.) voltage across 22, 2!, but this effect canbe kept. negligible by using. a. sllfllciently' high! Screen resistor 26. The A. C. voltage obtained on plate 24 is transmitted across a capacitor 28 to the cathode=4l of the diode 46... A. C. willdjevelop on plate 44 a negative D. C, voltage. with respect to cathode 4l If this voltage becomes higher than the positive voltage existing on the midpoint of the. voltage. divider 46', 41, the grid 52 of tube 50 will become negative enough to cut. oii the plate current. of tube. 5!!- and cause relay 5,8; to. dropout. If, later the illumination. of the phototube in increases. above a certain limit, the A. C-.. impedance on grid, 22 will decrease, causingthe A. C. voltage appearing there to decrease. The

reduced grid A. C. will cause lower A. C. on plate 24 and lower D. C. developed in 49, and consequently it will allow sufficient plate current in tube 50 to pull in relay 58.

The phototube I0, the grid 22 and the connection between them and the capacitor I5 has to be carefully shielded to avoid interference of spurious A. C. signals with the well defined A. C. injected to grid 22 by capacitor I5. The photoelectric current at which the relay will act can be varied by varying the capacitance of capacitor IS. The lower this capacitance is set, the lower will be the photoelectric current at which the relay acts. A limitation to the sensitivity of this circuit is set by the grid to cathode capacitance. Its susceptance is always parallel to the grid to cathode conductance caused by the photoelectric current flowing between grid and cathode. Therefore, the grid to cathode admittance will vary only slightly with variations in photoelectric current if the conductances due to these photo electric currents are below the susceptance of th grid capacitance.

i The sensitivity of circuits according to the invention may be increased beyond the above limit by a circuit as shown in Fig. 2. This circuit may measure accurately the extremely small current flowing in an ionization gauge I having an anode I4 and a cathode II. Those parts of the circuit which are similar to parts of the circuit shown in Fig. l, are denoted by the reference numerals as are used in Fig. 1. 29 is an amp1ifier tube having a cathode 2I, a grid 22, a screen grid 23 and an anode 24. 30 denotes another amplifier tube having a cathode 3I, a grid 32, a screen grid 33 and a plate 34. 29 denotes some means to measure alternating voltage, such as a cathode ray indicator or a vacuum tube voltmeter. 28 is a capacitor to withhold D. C. from the instrument 29. I8 is an instrument to measure D. C., conveniently of the Deprez type. I5 is a selector switch. 69 is a transformer, and I and 2 are, respectively, the negative and positive terminals of a source providing regulated D. C. voltage.

The primary 6| of the transformer 60 is connected to an A. C. source not shown and of very low frequency, say 5 cycles per second. Two equal secondary windings 62, 63 of the transformer 60 have a common terminal 65 connected to line I, whereas their opposite terminals 64, 61 are connected across capacitors I5, I5, respectively, to the grids 22 and 32 of the tubes and 30; a tap, 66 of the winding 63 is connected to one terminal of each of the capacitors II, I2, I3, I4. Switch I5 can make contact between the grid 32 of tube 30 and the other terminal of one of said capacitors II through I4. Grid 22 of tube 20 is further connected across a variable capacitor 68 to line I and across the ionization gauge II) to line 2. Grid 32 is further connected acros avariable capacitor 68" to line I, and across series connection of a fixed resistor I6, a variable resistor I1 and a galvanometer I8 to line 2. The screen grids 23 and 33 of the tubes 20, 36 are connected across capacitors 25 and 35, respectively, to line I, and across resistors 26 and 36, respectively, to line 2. The anodes 24 and 34 of the tubes 20 and are connected together and across resistor 21 to line 2. The indicator of alternating voltage 29 in series with the capacitor 28 is convalue, which thereinafter will be called preset grid capacitance and which may be, e. g. 5 'mmfd. The capacitors I5'and I5 are equal to each other and chosen so as to be a simple multiple or submultiple of said preset grid capacitance" or preferably equal to it. The tap 66 is taken at a point at which the voltage equals the voltage that would appear on grid 32 if switch I5 and resistor I6 were both disconnected. The capacitors II, I2, I3, I4 are chosen so as to have respectively a capacitance of say 99, 999, 9999, and 99,999 times the sum; of the capacitance of I5 and of the presetgrid capacitance. Each of these ratios increased by I will be called multiplier.

Measurement of the current flowing in the ionization gauge is done by altering the value of the resistor 11 until the indicator of alternating voltage 29 shows zero. If this is reached, then the reading of the galvanometer I8 divided'by the multiplier chosen by means of the selector I5, will give the current flowing in the ionization gauge I9. This result is based on the known facts that 1) a voltage divider is equivalent to a voltage source having the voltage appearing on the unloaded midpoint of the voltage divider and having an inner resistance equal to that which would be obtained by parallel connection of the two arms of the voltage divider, and (2) the current flowing from a thermionic cathode to the grid of a vacuum tube is an exponential function of the voltage between said two electrodes. From (1) it follows that grid 22 is influenced by an alternating voltage equal in magnitude but opposite in phase to that existing on tap 66, acting across a capacitance equal to the sum of capacitance I5'- and of the preset grid capacitance, whereas grid 32 is influencedby an equal voltage across a capacitance which is as many times as great as the former capacitance as the chosen multiplier indicates. To reach a balance between the plate signals of the tubes 20 and 39, it is necessary that the direct current injected across I0 to grid 22, and the direct current injected across I6, 'I'I, I8 to. grid 32, be in the same proportion as these capacitances. For only in this case will the additional conductances due to D. C. grid current of the grid to-cathode paths be proportional to the susceptances of the respective capacitive-voltage dividers.

The above discussion'assumed exact equality between the two tubes 20 and 36, and also that the resistors 26, 36 are high enough to keep the mutual conductance of both tubes equal despite the slight differences between their average grid voltages. Slight discrepancies from theoretical conditions can be taken into calibration or corrected by equalizing means as well known in the art. If thev theoretical conditions are not exactly adhered to, no exact balance can be obtained, and the meter 28 will indicate zero in no position of the resistor 11. Under such conditions, balance can best be found by utilizing an indicator that indicates the magnitude of only that component of the A. C. voltage difference between grids 22 and 32 which is out of phase with the voltage across 64, 61. Such an indicator indicates whether the resistor 11 is at one or at the other side of its position for best balance. A cathode ray tubenull indicator, e. g. one as described in my co-pendlng patent application Ser. No. 24,185 of April 30, 1948, may also be employed."

The frequency of the alternating voltage applied to transformer 60 should be chosen low, because the lower this frequency is, the higher will alternating voltage. The following numerical example illustrates the orders of magnitude in volved': With ordinary vacuum tubes the input capacitance may be held at Say mmfd, The re actance of 5 mmfd. at 5 C. P. S. is about 6000 megohms. The A. C. resistance of the grid to cathode path when a grid current of 10- amps. flows is in the order of 100,000 megohms.

The need for discrimination betweeniri phase and out of phase components of the. voltage ap= pearing on the anodes 24, .34 can be best explained in connection with the vectordiagram of Fig. 3. In this figure, the vector ABrepresents the alternating voltage that would appear between grid 2'2 and cathode 2| of the tube 20, if the grid current was zero. The vector AC rep resents the voltage between grid 22 andcathode 2| if, due to a certain grid current, the'grid admittance is increased. The vector DA represents the alternating Voltage between grid 32 and cathode 3| for zero grid current in tube 30, whereas the vectors EA, FA represent the alternating voltage between grid 32 and cathode 3| obtained with two different values of grid current. The point C lies on the are drawn around diameter AB; points E, F, and all other points corresponding to other values of grid current in the tube 30 lie on an are around diameter AD. Now, if thetap 55 is properly chosen, then the diameter AB coincides with the diameter AD, and the arcs drawn around these two diameters will coincide with each other. Therefore, whatever the position of point C may be, there can be found on are AFED a point coinciding with C. The direct current correspond ing to this point will cause perfect balance between the anode signals of tubes 20 and 30. But if the tap 55 is slightly different from what it should be theoretically, or if the stray capacitance from 22 varies after 68 had been properly adjusted, point D moves away from point B and there can be found no point on are AFED coinciding with point C. Then it is no more possible to obtain zero alternating voltage on anodes 24, 34. The minimum of alternating voltage will be reached .when the grid current in 30 brings the vector point on are AFED closest to point C. The plate signal will have a rather flat minimum, but its component perpendicular to the vector AB will go through zero almost exactly when this minimum is reached and this sharp zero reading may then be used to determine the position of point C, and from it, the current flowing between grid and cathode of the tube 20, which in turn is the current of the ionization chamber I0, desired to be measured.

When the multiplier is as high as in the examples given, the effect of the condenser 58" on grid 32 is negligible and 68" may be omitted.

In this circuit, balance had to be obtained manually. Automatic balance between the known current and the unknown can be obtained also by automatic means as is shown in Fig. 4 which rep-' resents" a circuit suitable, for instance, for the measurement of very low leakage currents flowing across parts to be tested for insulation. It comprises amplifier tubes 20, 30, Hand 50 each having respectively, a cathode 2|, 3|, 8|, 5|, a grid 22, 32, 82, 52, a screen grid 23, 33, 83, 53, and a plate 24, 34, 84, 54. There are further provided a twin diode 40', having cathodes 4| and 42- and anodes 44' and 43, a phototube 90, having a cathode 9| and an anode 94, a lamp 93, preferably of the glow discharge type, a D. C. microammeter I8, a transformer 60, and a'twin selector line 3.

l0 switch consisting of switch units 15 and I5. B supply is furnished by a conventional power supply not shown in the drawing, with its negative terminal connected to line I, its positive terminal of say 300 volts connected to line 3, and with its tap carrying regulated voltage of say 105 volts connected to terminal 2. The cathodes 2| and 3| of tubes 20 and are connected to line I. Grid 2.2 is connected across the fixed capacitor I5 to terminal 54, and grid 32 is connected across one of the capacitors H, I2, 13, I4 to terminal 56 of the secondary winding 62, 63, respectively, of transformer 60. The common terminal 65 of these windings is connected to line I. Grid 22 is further connected across one of resistors 6, 6, 6", and 5" and across switch 15 to terminal 5. Grid 32 is further connected across the phototube 90 and the microammeter I8 to line 2. The screen grids23 and 33 of the tubes 20 and 30 are connected across capacitors 25, to. line I, and across resistors 25, 35 to line 2. and 34 of the tubes 20 and 30 are connected together and across resistor 21 to line 2.

The cathode 8!, of tube 80 is connected across resistor 85 to line I, its grid .82 is connected across resistor 86 to line I and across capacitor 28 to the anodes 24 and 34 of tubes 26 and 30. The screen grid 83 is connected across capacitor 81 to line I and across resistor 88 to line 2. The plate 84 is connected across resistor 89 to line 2. The transformer 60 has a tertiary winding 69, whose midpoint 9'! is connected across capacitor 96 to the plate 84, and whose opposite ends 98, 99 are connected to the anodes 44' and 43' of the twin diode The cathodes 4| and 42' of 48" are connected across resistors and 45" to midpoint 91 0f the tertiary winding 69 of the transformer 50. The cathode 4| is further connected to line I and the cathode 42 is further connected to grid 52 of the amplifier tube 50. This grid 52 is further connected across capacitor 48 to line I. The cathode 5| of tube is connected to line I, its screen grid 53 to line 3 and its plate 54 across choke 92 and lamp 93 also to line 3. Ca-:

pacitor 95 is connected between plate 54 and The primary winding 6| of the trans, former is connected to a source of alternat-. ing voltage which may be of 60 cycles or of a substantially lower frequency, say below 25 C. P. S., if top sensitivity is to be achieved. The piece to be tested has to beinserted between terminals 4 and 5'. The resistors 6, 6', 6", 6" serve the doublepurpose of protecting the tube 20 in case of shortcircuit between the electrodes 4' and 5,,

;' and to prevent the capacitance of objects connected to electrodes 5 to interfere with readings. To have this second action efiective in all ranges of switch 15, a second deck '55 of the same switch selects automatically a resistor which will cause a voltage drop not above say 20 volts and not below say 1 volt for all current values to be measured'in the selected range.

The circuit shown in Fig. 4 operates as follows;

The leakage current flowing across the pieces inserted between the terminals 4, 5 will increase the admittance between cathode 2! and grid 22', and cause a change in the A. C. voltage on plates 24, 34. This signal is amplified in tube 80. Thevalues'ofthe'circuit elements 28, 35, 95, 45", and 45" are so chosen, that this voltage change is substantially in phase with thevo ltage existing in the" tertiary winding 69. The A. C. voltagesbetween anode M and cathode 4i" and between anode"43 and cathode 42' will then be the algebraic sum and the algebraic difference,

The plates 24 respectively, of half of the tertiary voltage in 60 and the alternating voltage on 84. Therefore, the D. C. voltage across 45 will increase and the D. C. voltage across 45" will decrease, or vice versa, and the grid voltage, and consequently, the plate current of the tube 50, will increase or decrease causing an increased or decreased lamp current and increased or decreased photoelectric current in 90, according to the magnitude and polarity of the A. C. appearing on plate 84. With proper polarity of the tertiary 69, this will tend to restore balance, and a new state of equilibrium will be reached in which the photoelectric current across 90 is substantially proportional to the current flowing between the terminals 4, 5. In this way, the microammeter 18 gives a direct information on the leakage current of the piece installed between 4 and 5.

In the foregoing embodiments, currents caused by physical quantities of different kind were sent through the circuit element having logarithmic voltage/current characteristics: The various physical quantities controlling small direct currents were compared against the alternating currents caused by a source of alternating voltage and a capacitor. In such applications the indication will vary if the sensitivity (i. e., current passed per unit of the respective physical quantity) of the circuit elements sensitive to the respective physical quantities varies. In the following embodiments, physical quantities will be compared with others of the same kind, and indication will be independent of Variations in sensitivity of the circuit elements sensitive to the physical quantities to be detected.

One such embodiment is shown in Figs. 5 and 6. It may be used to determine the spectral composition of a source of light. 20| denotes a source of light, e. g. a flame colored by chemicals, the composition of which is to be determined. 203 is a condensing lens, and 204 is a disc having four windows with color filters 205, 205', 205" and 205" in these windows. 206 denotes a shaft to which the disc 204 is fastened, and 201 denotes an insulating disc fastened to the same shaft 206. 208 is a conducting metal segment inserted into disc 201. I denotes a photo-tube having a cathode H and an anode I4. 209, 209, 209", and 209'" represent collector brushes. '20 represents again an amplifier tube having a cathode 2|, a grid 22, a. screen grid 23, and an anode 24, and 30 represents an amplifier tube having a cathode 3|, a grid 32, and an anode 34. 40' and 40" are twin diodes having each two cathodes 4|', 42' and 4|", 42", respectively, and two anodes 44, 43 and 44", 43", respectively, 50 and 80 are double triodes having each two cathodes 5|, 5|" and 8|, 8|", two grids 52', 52" and 82', 82 and two anodes 54', 54" and 84', 84". 18 and 18" are D. C. galvanometers. The circuit is fed from a supply of direct voltage not shown in the drawing which delivers a positive voltage of say 150 volts between lines 2 and I, and a negative voltage of say 150 volts between lines 2' and I.

The anode M of the phototube I0 is connected via line 2|2 to the positive line 2, and its cathode H is connected via line 2") to grid 22 of the first tube 20. This line 2|0 has to be carefully insulated, and it will preferably be held as short as feasible and shielded by a shield I from outside interferences. The connection of the tubes 20 and 30 is conventional with the one exception that there is no connection to grid 22, except by line 2|0, and that therefore the grid to trio current across ID. The output of the two stage amplifier consisting of tubes 20, 30 is ap-' plied across capacitors 49, 49', 49", and 49'" to the plates 43", 44", 43', and 44' of the twin diodes 40 and 40". These plates are further connected to the midpoints of voltage dividers 46, 41; 46', 41'; 46", 41"; and 46", 41'' respectively; the ends of which voltage dividers are connected to lines I and 2. The cathodes 4|, 42, 4|", are connected via resistors 45, 45', 45", and 45" to the positive line 2, and across lines 2| I, 2| 2| I", and 2| to one each of the brushes 209, 209', 209", and 209'". The conducting segment 208 is connected via line 2|3 to the line The anodes 54', 54", 84', and 84" of the twin triodes 50, are connected to line 2; their grids 52', 52", 82', and 82" are connected across resistors 51, 51', 51", and 51" to anodes 44, 43', 44", and 43" of the twin diodes 40' and 40'; and their cathodes 5|, 5|", BI, and 8|" are connected across resistors 59, 59', 59", and 50'" to the negativeline 2'.

The galvanometer I8 is connected between the cathodes 5| and 5| of the tube 50', and the galvanometer 18 is connected between the oathodes 8| and 8|" of the tube 80.

The operation of this device is as follows:

The shaft 206 is rotated at a speed of e. g. 250 R. P. M. during which rotation the filters 205, 205, 205", and 205" are interposed one after the other between the condenser lens 203 and the phototube l0. Simultaneously, the conducting sector 208 makes contact with the brushes 209, 209, 209", and 209", respectively.

The lens 203 concentrates light from the light source 20| upon the plane of the filters 205, 205', 205", and 205". The light, passed by that filter which at any particular time is positioned in front of the phototube, reaches the cathode H of this tube, and causes a certain photoelectric emission to the anode l4. In view of Kirchoffs law, a cathode to grid current equal to this photoelectric emission must flow from the cathode 2| to the grid 22 of the tube 20. Grid 22 will there-. fore assume that potential at which this particular value of grid current passes. The variations of this voltage difference are amplified by the tubes 20, 30 and are applied via the condensers 49, 49, 49", and 49" to the plates 43", 44" and 43, 44' of the twin diodes 40 and 40". The time constant of the condenser 28, resistor 38 combination as well as the time constants of the condensers 49, 49, 49", and 49'", with the coordinated resistors 46, 41, 46', 41', etc., and is chosen high enough to keep the diode anodes 44,

43', 44", and 43" at a substantially constant po-' tential for the duration of the passage of each filter 205, 205', 205", and 205" in front of the phototube 0.

According to the spectral composition of the light source 20| and the selectivity of the filters 205, 205', 205", and 205'", the time curves of voltage on the diode plates will assume shapes as are shown schematically by lines H of Figs. 7 and 8. On Fig. 7, 0 represents the voltage of line used as a, reference, J the voltage of cathode 4|, II that of anode 44'. M is the average of line H. On Fig. 8 the same letters refer to the same curves of cathode 42' and anode 43'. The curves H of Figs. 7 and 8 are similar to each other. They differ from each other only in their average height, this height being determined by the action of the diodes in the following manner:

The diode cathodes are held'for most of the must equal the photoelec-i and 42" of these twin diodes 40' and 40" 13 time at the potential of "line 2 via resistors 45, 45, 45", and 45". 'With the'cathodes held at this positive potential, no current can flow in these diodes. During the passage of each color filter in front of the phototube, however, one of the diode cathodes is shortened to line I across one of the brushes 209, 209,- 20-9, and 209'. In these periods, the respective-diode systems become conductive and the respective anode will assume a voltage close to that of its coordinated cathode. The height of the voltage curve during this period will determine the average height of the entire voltage curve. As a consequence, grids 52', 52" of the twin triode 50', which are com nected to the anodes 44, 43 of the diode 40, as sumo a voltage difference depending on the logarithms of the proportion of the photoelectric currents flowing through the tube during the passage of the two filters 205' and 205, and the current across the meter 10' dependsin direction and magnitude on the proportion between light transmitted in'these two periods. Meter 18" will indicate in a similar way the proportion between the light transmitted by filters 205' and 205'.

Since the instrument is sensitive only to the f proportion between these components of the light emission, it will give true indication irrespective of variations in the intensity :of the'fiame, ageing of the phototube, or any other causes that affect all components of the light in the same proportion. One of the filters may be gray; colorless, or may be entirely omitted, in order to have as a reference the total light intensity of the flame or a desired percentage of it. Any number of filters and any number of double diodes may be used. In the example given in the drawing, both filters 205 and 205' are compared with filter .205. Filter 205" is not used. It could be omitted entirely, it could be compared by a third twin diode with filter 205, or whilst 205 is compared with 205', 205" could be compared with .205. One grid might also be connected to a voltage divider between two or more diode anodes, in order to obtain an average depending on two or more color filters. By varying the ratio of the divider arms, any desired weight can be given to each filter.

In the above example, the instrument was used to indicate the color composition of a flame, but the same circuit may .be used to give exact numerical data on the color of a reflecting or a transparent materialby illuminating it by a suitable light source and determining; the proportion between the components falling within certain wave length limits of the transmitted or reflected light.

If only the shade or particular variety of the color is to be determinedthen an optical arrangement substantially similar to that of Figs. 5 and 6 may be used, with the sample to be tested in place of the flame but if also the density of the color is to be determined, then it is advantageous to use, as a reference, some light of the source that is not reflected from or' passed through the material to .be tested. Fig. 9 shows a schematic optical arrangementcfor this purpose which may be used with an amplifier similar to that shown in Fig. 6. Here 20l denotes alight source which may be the filament-otan incan: descent lamp 202. 2H indicates the surface of the material the light reflection from which is to be tested; 203 is a lens; 304 is a disc which can be rotated by its shaft .205, 2 l6 a second lens, and W a phototube having a cathode H and an anode not shown. 2 I8 is a spherical mirror. 2 I9 is another spherical mirror. 'lhe disc 304 has a c flat portion and one bent segment. The, flat po'rtion contains a number of windows for insertion of color filters 205; the bent portion locates a plane mirror M5. 220 denotes an aperture stop.

The surface 2!! is illuminated by lamp. 202, The light reflected from it is gathered by lens 203 in the plane of the filters 205. Lens 203 is imaged by lens 2H3 Onto the photoelectric cath ode I l. During the period in which the bent portion of the disc 304 passes between the lenses 203 and 210, the light reflected from surface 2H can not reach the phototube, there being no passage through the inclined mirror 2l5. However, light falling on mirror 2! is reflected by mirror 219 onto the mirror 2l5, which directs it across. lens .210 to the photoelectric cathode ll. Thus, whereas during the passage of the filters 205 between the lenses 203 and 216,, the p'hototube is illuminated by the various components of the light which is reflected from 211, the phototube is illuminated by an amount of light depending only on the light emission of the light source and of the setting of the aperture stop 220 during the passage of mirror 2l5 between the same lenses. In this manner, there may be compared the light reflection .in desired bands with an adjustable amount of light emitted from the lamp. The galvanometers of the instrument may thus determine directly the color of materials in accepted system of color determination without the need of visual color matching. The readings will again be independent from variations in light intensity of the lamp. To prevent variations in the color of the light emitted for extremely exact measurements, or where the line voltage is very unstable, the voltage applied to the lamp can be kept constant by some voltage regulating transformer.

In the two preceding embodiments, more than two physical quantities of the same kind were compared with each other. In the embodiment of Fig. 10, only two .physical quantities of the same kind are compared with each other. In this figure, which includes a circuit I have disclosed in my co-pending application Ser. No. 58,423, 2M denotes the filament of an incandescent lamp 202. 218 and H8 are spherical mirrors, and 22l and 22V are containers for a liquid to be tested and a reference liquid respectively, these containers having transparent walls 222. E0 denotes a phototube having a cathode II and an anode I4. 40 3 is a disc rotatable on its shaft 205. 20, 30, and denote amplifier tubes, having cathodes 21, 5!, 5i, grids 22, 32, 52, screen grids 23, 53, and plates 34, 2t, 54, respectively. 40 is a diode having a cathode 4| and an anode 44. 2&0 is a voltage regulator tube, having a cathode 2 and an anode 244. 5'8 denotes a relay having a contact 241, whereas 246denotes some signaling device, and 248, 249 are the terminals of a source of voltage.

The filament 20! is imaged by mirrors 218, 218' inside the fluid to be tested and inside the stand- 1 ard fluid, and proceeds from there towards the phototube :0. The light passing the container 221 is uninterrupted, but the light passing the container 22V is periodically interrupted by blades or a rotating disc 404 which may be of the kind shown in Fig. 12, to be described later. Line i is the negative and line 3 is the positive terminal of a source 01? direct voltage (not shown) of say 300 volts. Resistor 245 reduces this voltage to say volts on line 2, held constant by the voltage regulator tube 240. The anode I4 0! 15 phototube I'D is-cjonnected via lead 2| 2 to line 2', and its cathode H is connectedvia lead 2) to [grid 22 of tube 20. Line 2! is carefully shielded by shield "l. The cathode 2| of tube 26 is connected to line I, and its anode 24 via load resistor 21 to line 2. The voltage on screen 23 is maintained by resistor 26 and capacitor 25, the latter being large enough to keep the screen voltage substantially constant when the plate current varies due to variations in photoelectric current caused by the interruptions of light by disc 404. The output of plate 24 is brought onto grid 32 of tube 38 via capacitor 28. This grid 32 is'connected to line across a conventional grid resistor 38. Parallel to this grid resistor there is'connected capacitor 225 in order to bypass alternating'volta'ges of substantially higher frequency than that of the interruptions efiected by disc 404. Cathode 3i is connected to ground, via a conventional cathode resistor 39, which may be bypassed by a capacitor not shown in the drawing. The load resistance of tube 30 is composed of a fixed resistor 31 and a voltage divider 22T'having a variable tap 228. Capacitor 226 serves to bypass'that portion of undesired frequencies which persists despite capacitor 225. The tap 228 is connected across capacitor 242 to a voltage divider consisting of a series connection of resistors 229, 230, 23l, and 232. Switch 233 connects a desired tap of this voltage divider to the anode 44 of diode 40. This diode anode is further connected across resistor 48 to grid 52 of tube 50. The cathode 4| of the diode 40 is connected to line I, and so is the cathode of tube 50. Capacitor 56 is connected between grid 52 and line I, screen 53 is connected to line 2, anode 54 is connected across the coil of relay 58 to line 3. The signalling device 246 is connected to the source of voltage 248, 249 across the contact 241 of relay 58. 4

The operation of this device is as follows:

The phototube I0 is illuminated by both the steady flux passingcontainer 22! and the periodically interrupted light flux passing container Hi. The photoelectric current passing from cathode II to anode I4 is the sum of the currents caused by these two types of illumination, and it will have a lower limit determined by the light passing container 22| alone and an upper limit determined by the light passing both the container 22| and the container 22l'. In view of Kirchofis law,- this same current passes from cathode2l to grid 22. As shown earlier, the voltage variation on grid 22 caused by this current of varying magnitude (composed of the constant photoelectric current caused by the light passing container 22| and the varying photelectric current caused by the light passing container 22|') is proportional to the logarithm of the proportion between the upper limit of the current and the lower limit of the current. It is therefore independent of variations of the light emission of the filament 2ll| or the sensitivity of the photocathode II. For highest accuracy, care should be taken to have the same portions of the cathode exposed to both these light fluxes and to make the angles of incidence from both light sources equal to each other. The alternating voltage obtained on grid 22 is amplified in tubes 20 and 30 and a desired portion of the amplified voltage is applied to the diode 40. When this portion of the voltage on the anode 34 reaches a certain limit, the grid 52 becomes-negative enough to out off the plate current of tube 50 and cause the relay '58 to close contact 241 and apply operating volts egisifisti age from source 248 249 to the signalling device 246. I

The accuracy of the system just described can be increased if, instead of applying one physical quantity continuously and the other intermittently, the two-are applied alternatingly and not only the amplitude of the alternating voltage obtained on the logarithmic element, which is a measure of the percentage deviation, is detected, but also its phase relationship with respect to the interruptions, which indicates which of the two physical quantities is greater. Such a system is shown in Figs. 11 and 12. In these figures, those parts that are similar to parts of earlier figures are designated by. the numbers used in those earlier figures and, in addition thereto 504 is a disc so arranged that when one of said light fluxes is interrupted, the other is uninterrupted, and vice versa. The periods of full interruption and of full passage are lengthened, and the periods of transition are shortened by focusing an image of filament 2M on the plane of the disc 504.

The circuit comprises an additional phototube l0 having a cathode H" and an anode l4". This phototube is illuminated from filament 2! by light reflected from a spherical mirror 2|8. This mirror focuses the image of filament 2! on the same spot on which it is focused .b mirror 2 It, so" that interruption of the light proceeding toward phototube |0"' occurs simultaneously with the interruption of light passing container 22 The operation of this device is as follows: The phototube I0 "conducts one periodically varying photoelectric current caused by the periodically interrupted light-across 22| and another periodically varying current caused by the periodically interrupted light across 22l. These two currents may add to one continuous current free of periodic changes in the special case that the two light fluxes are identical, but usually their resultant is a more or less modulated current, the polarity of modulation depending on which of the two light fluxes is stronger. The photoelectric current flowing across l0 passes from cathode 2| to grid 22 of tube 20, and causes between these electrodes a voltage drop whose instantaneous values are in linear relationship to the logarithm of the instantaneous value of the photoelectric current) The alternating component of this voltage is amplified in tube 20 and brought to the grid 252 of a phase splitting tube 250. On the cathode25| and on the anode 254 of this tube alternating voltages are obtained substantially equal to each" other and to the output voltage obtained on anode 24, but being of opposite polarity. Thesetwo voltages are applied to the cathodes 4|",an'd42 of a twin diode 40'. The photoelectric current'of the phototube 10" is conducted across a conventional load resistor 265 and is amplified in a conventional manner in tube 30. 'The output of this tube is applied across capacitors 260 and 26| to both anodes 44, 43' of the twin diode 40'. The circuit elements of the various amplifier stages are so chosen that the signals obtained on theanodes 44', 43" are in phase with the signal obtained on cathode 4| and in opposite phase with the signal obtained on cathode 42', or vice versa, according to whether the photoelectric current caused by the light across 22! is greater or smaller than the photoelectric current caused by" the light across container Hi. The desired relationship can be achieved either by making the time constants-of the resistance capacitance combinations so high that. phase shift dueto them is negligible, or by 17 choosing them so that the phase shift in the amplifier train from |8 to 4!! equals that of the phase shift in amplifier train from Hi" to. 4D;

the difference of the outputs of tubes 20 and 30.

The rectified voltage appearing across resistor 262 will be above or below that appearing across resistor 263, according to whether the light passing the container 22! is above or below that passing the container 22!. The magnitude of this voltage diiference is a, measure of the percentage deviation between these two fluxes of light. The difference between the voltages across the two equal resistors 262 and 263 causes a current across a limiting resistor 264 and microammeter 18, which may be calibrated directly in plus and minus percents of transmission of light across one fluid compared to the transmission across the other fluid. r

In the preceding embodiments, the inconstant current varies periodically. An example for detecting a physical quantity by adding to the current caused by it one single pulse caused by, a physical quantity of the same kind and of known value is given in Fig. 13. It represents an instrument to measure the ionization present in a cer- I tain locality in terms of ionization from a reference source of ionization 2-83. In this figure, l8 denotes an ionization gauge having a cathode I and an anode l4, and 20 is an amplifier tube having a cathode 2!, a grid 22, a screen grid 23, and an anode 24. A source of direct current, not shown in the figure, suppliesa voltage of say 300 volts, which may be kept constant by a, series connection of two voltage regulating tubes, likewise not shown. Line is the negative and line 2 is the positive terminal of this source. The anode !4 of the ionization gauge !8" is connected to line 2, whereas its cathode! I is connected to grid 22 of tube 20. Cathode 2! of tube 29 is connected to line I, screen 23 is connected to line 2 across a resistor 26 of say 3 megohms and to line across an oil filled condenser 25 of say 2 microfarads. Plate 2 is connected to the midpoint of a voltage divider 28!, 282 across the meter '18 having a full scale deflection of say 50 microa-mperes, and it is connected to line 2' across the variable resistor 280. 283 is somesourceof ionization. It maybe a small generator of Xrays,.a wellshieldedquantity of some radio-active material, or any other convenient source of radiation. 284 symbolized means to start or stop action Of such generator of ionization on the gauge In the case-of an X-ray generator, it may be a switch to apply platepower, in the caseof a radioactive-material, it maybe the lever to a shielding window; the opening of whichallows-the radiationtoact on thegaugelil.v r I g The operation .of the apparatusis the follow.- ing: .Before making a measurement the operator adjusts the resistor 280 so th'at the meter -;18.-reads zero. ,He vthen depresses lever 284and readsthe deflection of the instrument. This deflection will be ameasure of ambientionization. Thisaction is based on the following facts: 3 I The ionization present in the locality will cause a, certain ionization currentto flow in Inf, and this ionization current will pass from cathode .2! to grid 22. This current willcause a certainvoltage drop to exist between these electrodes ;2.|,;22- The. current across resistor 26 ,will assume-a sta- .tionary value at which the voltage on screen grid 23 will be as high as is required to cause this current to fiow from cathode 2! to screen 23, with the voltage on grid 22 as high as determined by the ionization current in !0. If now, the ionization current suddenly varies, the screen voltage will vary very slowly, due to the high time constant of combination 25, 26. The plate current will vary abruptly by an amount significant of the change in grid voltage. The variation in grid voltage, in turn will be dependent on the loga- 'rithm of the proportion between ionization current before activating 2'83 and ionization current after activating it, and thus the swing of the meter 18 in consequence of the depression of lever 284 is an indication of the proportion between theambient ionization to be determined and the additional ionization caused by generator 283.

If the physical quantity to be detected is some transmission property of a certain material, then this material may be periodicallyinterposed as shown in Figs. 14 and 15. They represent a device for measuring the transmission of gamma radiations across a certain thickness of a material. In these figures, 298 represents a rotatable platform comprising two open containers '29 I, 292 into one of which'thematerial to be tested isto be poured. 283 is a generator of gamma radiations, and I0 is an ionization gauge having a cathode H and an anode !4'. The platform 290 .is rotated around the axis of its shaft 205.

The radiation from 283 to H1 is intercepted for part ofthe time by the empty container and for part of the time by the container which contains the material to be tested. Accordingly, the currents in ID will vary between an upper limit set by the transmission of the empty container and a lower limit set by the transmission of the full con tainer. The voltage variations between '22 and 2! are, as described before, proportional to-thelog-a rithm of the ratio of the two levels of transmitted radiation. These voltage variations are amplified by the tube 20,-rectified by the diode 40, and indicated by the meter 18.

The one of the two containers 29!, 292 being filled with the material to be tested, the other container may be filled with a material to be used as a reference. .In that case, the circuit shown would indicate only the proportion between the radiations obtained in the two periods, but not which of the two materials has the higher absorption. For certain purposes, this may be adequate, such as where it is definitely known whichof the two materials has the higher ab sorption, or if it is only desired to check uniformity irrespective of the direction of deviation, but for other purposes it will be preferable to provide a monitoring circuit, e. g. on the principles underlying Fig. 6 or Fig. 11, adapted for indication of the direction of the deviation.

It will be noted that whilst in previous art, the currents to be detected by an amplifier tube were conducted across a load resistor and the voltage across said load resistor was applied between the grid and the cathode, or preferably between the grid and a point negative withrespecttothe cathode, I conduct the currents to be detected across the grid to cathode path of the amplifier tube anddetect the voltage drop caused by these currents across this path. For this purpose, it is necessary to conduct said currents in the direction in which the grid to cathode path is conductive. It is also necessary that the electrode that is being connected to the grid be insulated from the cathode in order to prevent other points of the circuit connected to the oathode across galvanic circuit elements would create such bypass which is undesired here] The other electrode of the circuit element that passes the current to be detected will conveniently be connected to the cathode across circuit elements adapted to conduct direct current to complete the circuit. These circuit elements adapted to conduct direct current might include a source of voltage and, if desired, resistors for the purpose of limiting orfiltering the current. But I have found that in many cases I do not need the source of voltage just mentioned. For example, in-a circuit as shown in Fig. 10, satisfactory results may be obtained when connecting the anode i 4 of the phototube e. g. a type 929 phototube to line I. I arrived at good results even when disconnecting the anode l4 entirely. It may be presumed that in these cases, leakage currents between anode I 4 and line I' complete the circuit. As a, rule, however, galvanic connection across well determined circuit elements will give more reliable esults. The voltage between the grid and the cathode will appear in series with other electromotive forces in the circuit coritaining the circuit element the current through which'is tobe detected. Therefore, thiscircuit element should be chosen so that said voltage between grid and cathode and variations thereof will not adversely afiect the accuracy and linearity of the response of said circuit element. 7

While I have illustrated in the drawings and described in" the foregoing description several specific embodiments of myinvention I desire it to be understood'that these embodiments have been given by way of example only since'the illustrated circuits and structural elements'may be modified and. interchanged in many ways which will be obvious to those skilled in the art of applied electronics. Furthermore, the invention is not limited to such applications as are shown'in the drawings, the illustrated photometers, spectrometer, ionization gauge, colorimeter, etc., being only illustrative for the wide applicability of the principles'of the invention, many other applications being possible without depart ing from the spirit of the'invention or the scope of the appended claims.

- What I claim is:

1; Amethod of comparingtwo or more physical quantities, comprising the steps of causing electric'curre'nts whose magnitudes are functions of said physical quantities and of which one at least is incons'tant' in such a. manner as to vary its instantaneous value by an amplitude depending on'xlthe magnitudeof the coordinatedfphysical quantity to flowsimult'aneously between the electrodes "of such a circuit element that a'current within the range of the momentaryvalues of the algebraic'sums of said currents passing through said circuit element will cause a voltage between said electrodes which is a logarithmic function of such current as passes between said electrodes, and detecting the variations of voltage occurring between said electrodes at the rate of the varia-- tions of the instantaneous value of said inconstant current. f

- 2. A method of comparing two or more physical quantities through the intermediary of a vacuum tube having at least one thermionic cathode and at least-one other electrode, the electron emi's s'ion to which from said cathode is temperature,

2o limited, comprising the stepsof causing electric currents whose magnitudes are functions of said physical quantities and of which one at least is inconstant in such a manner as to vary its i.n stantaneous value by an amplitude depending on themagnitude of the coordinated physical quantity, to flow simultaneously between said cathode and'said other electrode of said vacuum tube, and detecting the variations of voltage occurring between said cathode and said other electrode of said vacuum tube at the rate of the variations of the instantaneous value of said inconstant current.

3. A method of detecting a physical quantity by comparison with another physical quantity of the same kind, at least one of said physical quantities being inconstant in such amanner as to vary its instantaneous value by an amplitude depending on the. magnitude ofthe coordinated physical quantity, comprising the steps of causing both. said physical quantities to influence simultaneously a circuit element so as to pass through it a current whose magnitude is proportional tothe sum of the instantaneous values of said two physical quantities, causing the current across said circuit element to flow across the electrodes of another circuit element having at least two electrodes the voltage across which is a logarithmic function of such current as passes across these electrodes, and detecting the voltage variations across said electrodes of said other circuit element occurring at the rate of said variations of the instantaneous value of said incon-- stant physical quantity. i

4. Amethod of detecting a physical quantity .by comparison with another physical quantity of the same'kind, through the intermediary of a vacuum tube having at least one thermionic cathode and one other electrode the electron emission to which from said cathode is temperature limited, at least one of said physical quantities being inconstant in such a manner as to vary its instantaneous value by an amplitude depending on the magnitude of the coordinated physical quantity, comprising the steps of causing both said physical quantities to influence simultaneously a circuit element so as to pass through it a current whose magnitude is proportional tothe sum of the instantaneous values of. said two physical quantities, causing'th'e curre'ntacross said circuit element to flow between said thermionic cathode and said other electrode of said vacuum tube, and detecting the voltage variations between said thermioniccathode and said-other electrode occurring at therate of said variations of the instantaneous-value of said inconstant physical quantity; I 5. A method yofdetecting "a first physical quantity by comparison with a second physical quantity of the same kind through the intermediary of a first electric circuit element having at least two electrodes the voltage acrosswhich is a logarithmic i unction of such current as passesacross these electrodes; comprising the steps oi varying the magnitude of] one of said physical quantities between an upper limit and a lower limit, causing both said physical quantities to influence simultaneously a second circuit element so as to pass through it .a current whose magnitude is proportional to the sum of the instantaneous values of said two physical quantities, causing the current across said second circuit element to how across said electrodes of said first circuit element, and "detecting the voltage 21 variations across said electrodes of said first circuit element occurring at the rate of said variations of the magnitude of the oneof said physical quantities.

6. A method of detecting a first physical quantity by comparison with a second physical quantity of the same kind through the intermediary of a vacuum tube having at least one thermionic cathode and one other electrode, the current to said other electrode from said thermionic cathode being temperature limited,

comprising the steps of varyingthe magnitude of one of said physical quantitiesbetween an upper and a lower limit, causing both said physical quantities to influence simultaneously a circuit element so as to pass through it a current whose magnitude is proportional to the sum of the instantaneous values of saidtwo physical quantities, causing the current across said second circuit element to flow between said cathode and said other electrode of said vacuum tube, and detecting the voltage variations betweensaid cathode and said other electrode occurring at the rate of said variations of the magnitude of the one of said physical quantities.

7. A method of detecting a first physical quantity by comparison with a second physical quantity of the same kindand known magnitude through the intermediary of a first electric circuit element having at least two electrodes the voltage across which is a logarithmic function of such current as passes acrossthese electrodes, comprising the steps of varying the magnitude of said second physical quantity between known limits, causing both said physical quantities to influence simultaneously a second circuit element so as to pass through it a current whose magni; tude is proportional to the sum of the instantaneous values of said two physical quantities, causing the current across said second circuit element to flow across said electrodes of said first circuit element, and detecting the voltage variations across said electrodes of said first circuit element occurring at the rate of said variations of the magnitude of said second physical quantity.

8. A method of detecting a first physical quantity by comparison with a second physical quantity of the same kind and known magnitude through the intermediary of a vacuum tube having at least one thermionic cathode and one other electrode, the current to said other electrode from said thermionic cathode being temperature limited, comprising the steps of varying the magnitude of said second physical quantity between known limits, causing both said physical quantities to influence simultaneously a circuit element so as to pass through it a current whose magnitude is proportional to the sum of the instantaneous values of said two physical quantities, causing the current across said circuit element to flow between said cathode and said other electrode of said vacuum tube, and detecting the voltage variations between said cathode and said other electrode occurring at the rate of said variations of the magnitude of said second physical quantity.

9. A device to detect a physical quantity, comprising a circuit element adapted to pass a current proportional to the physical quantity to be detected, said circuit element having two electrodes, a vacuum tube having at least one thermionic cathode and one other electrode, a source of alternating voltage of given frequency, means for detecting the alternating voltage of said frequency between said thermionic cathode and said other electrode, one of said electrodes of said circuit element being connected to the first of said other electrodes of said vacuum tube and insulated from said thermionic cathode; the other electrode of said circuit element being-com nected to said thermionic cathode across circuit elements adapted to conduct direct current, said source of alternating voltage being arranged in series with a capacitance, and said series connection of voltage source and capacitance being connected between said thermionic cathode and said other electrode of said vacuum tube. V

e 10. A device according to claim 9 whereinthe source of alternating voltage has a frequency below 25 cycles per second.

' 11. A device for detecting a physical quantit comprising a circuit element adapted to-pass a current proportional to the physical quantityto be detected and having two electrodes, a vacuum tube having at least one thermionic cathode, one grid and one anode, said anode of said vacuum tube being connected to said cathode of said vacuum tube across a path adapted to conduct direct current and having alternating current impedance, a source of alternating voltage of a given frequency, means for detecting the alternating voltage of said frequency on said anodeof said vacuum tube, one of said electrodes of said circuit element being connected to said grid of said vacuum tube and insulated from said thermionic cathode of said vacuum tube, the other electrode of said circuit element being connected to said thermionic cathode of said vacuum tube through circuit elements adapted to conduct direct current, said source of alternating voltage being arranged in series with a capacitance, said series connection of voltage source and capacitance being connected between said thermionic cathode and said grid of said vacuum tube, and means for injecting an adjustable amount of alternating current of said frequency into said path.

12. A device as claimed in claim 11 and having means for automatically controlling the amount of alternating current injected into said path adapted to conduct direct current and having alternating current impedance in dependence on the alternating voltage of said anode, and means for detecting the magnitude of the automatic action of said control means.

13. A device for determining the proportions between a number of physical quantities of equal kind, comprising an electrical circuit element sensitive to the kind of physical quantities to be determined, means to cause said physical quantities to alternatingly act upon said circuit elemerit, a vacuum tube having at least one cathode and one other electrode, means for detecting the difference between the voltage differences between said cathode and said other electrode during action of at least two of said physical quantities on said circuit element, one electrode of said circuit element being connected to the first of said other electrodes of said vacuum tube and insulated from said cathode of said Vacuum tube, and the other electrode of said circuit element being connected to said cathode of said vacuum tube through circuit elements adapted to conduct direct current.

14. A device for comparing a first physical quantity with a second physical quantity of equal kind, comprising an electrical circuit element sensitive to said physical quantities, a vacuum tube having at least one cathode and one other trodes and said cathode of said vacuum ,tube, one

electrode of said circuit element being connected to the first of said other electrodes of said vacuum tube and insulated from said cathode of said 1 vacuum tube, and the other-electrode of said circuit element being. connected to said cathode of said vacuum tube through circuit elements adapted to conduct direct current.

- 15. A device for comparing a first physical quantity with a second physical quantity of equal kind, comprising an electrical circuit element sensitive to said physical quantities, a vacuum tu be'having at least one cathodeand one other electrode, means to alternatingly interrupt the action of said physical quantities on said circuit 1 element, means adapted to detect magnitude and direction of the variation of voltage occurring between an interruption of the action of one of said physical quantities on said circuit element and the following interruption of the action of the other of said physical quantities on said air-- 'cuit element, one electrode of said circuit element being connected to the first of said other electrodes of said vacuum tube and insulated .a source adapted to generate a second physical quantity of the same kind as thatto be measured, means to abruptly initiate action of saidsource on said circuit element for abruptly changing the voltage ;=betw,een said other electrode and said cathode of said vacuum tube, means for measuring said abrupt voltage changes, one electrode of said circuit element being connected to the first of said other electrodes of said vacuum tube and insulated from said cathode of said vacuum tube, and the other electrode of said circuit element being connected to said cathode of saidvacuum tube through circuit elements adapted to conduct direct current. I 1

17. A device for measuring a first physical quantity, comprising an electrical circuit element sensitive to said physical quantity, a vacuum tube having at least one cathode and one other electrode, a source adapted to generate a second physical quantity of the same kind as that to be measured, means to abruptly interrupt action of said source-on saidcircuit element for abruptly changing the voltage between said otherelec trode and said cathode of said vacuum tube, means for measuring said abrupt voltage changes, one electrode of said circuit element being-connected to the first of said other electrodes ofsaid vacuum tube and insulated from said cathode of said vacuum tube, and the other electrode of said circuit element being connected to said cathode of said vacuum tube through circuit elements adapted to conduct direct current.

' JOSEPH C. FROMMER.

1 REFERENCES CITED The following referencesare of record in the file of this patent:

Number 

